The interference may not be the direct cause of the drone but should be corrected anyway. Contact like that may cause vibrations at all times. I had an ear on a transmission crossmember rest against the trans mount and it buzzed at all times! The engine and trans, even when perfectly balanced, produce vibrations that certainly will find their way to you if they are not absorbed by something. The rubber in the mounts does this. This is also why the poly urethane mounts transmit more vibration.
I'd rather see the header clearanced (Dented) than to mess with altering the steering box position. The latter may result in increased bump steer, steering wheel off center, more turns to left than right. (Or vice-versa)
Hot Rod magazine did a dyno test where they tested an engine with new header,s then they bashed several tubes. The second test showed only a very slight loss in power. I took that to mean that one should not be afraid to put a few dings in their headers to get the clearance that you need. It sucks to ding up new pipes but life happens...
If you have turn down exhaust pipes in front of the rear axle, you will be amazed at how much quieter the car would be with full tailpipes that exit beyond the rear bumper by an inch or so. The under car exit causes the sound waves to resonate, reverberate and buzz the entire car. I don't have any experience with the type of mufflers that you have. I ran Flowmasters for many years but changed to Dynomax welded Ultra Flo mufflers to eliminate the echo/reverberation. These Ultra Flos haxe zero drone at any rpm or road speed. I know that you have spent a ton of $$$ and may be looking to save a few bucks. I know I would. You can try dimpling the header for clearance to see if it helps. I doubt that it will but it is needed anyway. Shoot for at least 1/4" clearance.
